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Liquid Crystal Devices

Liquid crystal (LC) is a unique substance that shares properties of both liquids and solid crystals. It can flow like a liquid, but its molecules can be organized like a crystal. There are many kinds of LC phases, which we can tell apart by their optical properties. The differences in these properties are due to the orientation of the molecules within the material.

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Not all LC materials will always be in an LC state, just like water can be in the form of ice or vapor. There are three main types of liquid crystals: thermotropic, lyotropic, and metallotropic. The simplest kind of liquid crystal phase is the nematic phase. In this phase, the molecules align parallel to each other but are free to flow and their positions are randomly distributed, similar to a liquid.

 

The chiral nematic phase is another type of LC phase that shows chirality, or handedness. This was first seen in cholesterol derivatives, so it’s often called the cholesteric phase. Only chiral molecules can create such a phase. In this phase, the molecules twist in a direction perpendicular to the director, which results in a longer-range chiral order.

In the smectic C* phase, the molecules are ordered in layers and are tilted at a certain angle. This tilt creates a spiral twist of the molecular axis, so they are also known as twisted nematics.

 

Chiral liquid-crystal molecules usually create chiral mesophases, meaning the molecule must have some form of asymmetry. But the system can’t be racemic, meaning it can’t be a mix of right- and left-handed molecules, because that would cancel out the chiral effect. However, adding a small amount of chiral dopant to an achiral mesophase can make the system overall chiral.

Chiral phases usually have a helical twist of the molecules. If the twist is about the same size as the wavelength of visible light, it can create interesting optical interference effects. The twist in chiral LC phases also makes the system react differently to right- and left-handed circularly polarized light, meaning they can be used as polarization filters.

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